The present invention relates in general to diesel engine filtration systems and in particular to a coalescing filter to remove oil aerosol from a blowby gas (exhaust) stream. More specifically, the present invention relates to a coalescing filter which is subjected to rotation in order to expel the coalesced liquid from the filter and thereby keep any flow restriction within the filter comparatively low.
The present invention focuses on the addition of an air/oil coalescing filter as part of a rotating lube bypass centrifuge in order to remove oil aerosol from blowby gas associated with an internal combustion engine crankcase ventilation system. The coalescing filter is subjected to high-speed rotation which assists in expelling the coalesced liquid (oil) from the filter. This in turn helps to maintain a low filter restriction and a low crankcase pressure.
In order to achieve high separation efficiency for oil aerosol in the 0.1–1.0 micron size range, it is necessary to use a relatively “tight” coalescing medium which is constructed from very fine fibers (melt-blown or glass). A consequence of fine fibers is the corresponding fine pore size distribution. The presence of fine pores in a coalescing filter can result in the pores becoming “clogged” with the liquid being separated, due to the surface tension and the corresponding “bridging” effect. This relatively high surface tension causes a correspondingly high restriction since it takes a large pressure to overcome the surface tension across a small wetted pore. It is known that the pressure required to “blow out” a pore is inversely proportional to the pore diameter. This behavior has been clearly verified by testing with various grades of media. What has been learned is that the pressure required to break through the film of a wetted pore is several times higher than the “dry” restriction at design face velocity. The lowest reported difference in wet flow restriction compared to dry flow restriction was a 3-fold increase in flow restriction for the wetted condition.
Since engine crankcase pressure must be kept very near atmospheric pressure, approximately 5 inches of water, it is difficult to design a high-efficiency coalescer without resorting to a fairly elaborate arrangement of pressure control valves, vacuum assist devices, and similar mechanisms. For this reason, a means of keeping the coalescer element dry and operating at a low restriction is important for any useful improvement.
This technology has heretofore been utilized in integrating a coalescing filter with a rotating component, specifically a gear within a gear housing, as described in U.S. Pat. No. 6,139,595 which issued Oct. 31, 2000 to Herman, et al. U.S. Pat. No. 6,139,595 is hereby expressly incorporated by reference for its entire disclosure. However, prior designs such as that disclosed in the '595 patent, where the coalescing filter is mounted to a structure such as a gear, have had their performance limited to some degree due to the rather low speed of the rotating component, such as one half of the engine speed. The present invention overcomes that limitation by mounting the coalescing filter to a component with a much higher rotative speed, specifically a lube system centrifuge rotor.
Higher rotative speeds increase the “cleaning effect” that is seen in the coalescing filter element, as described in the '595 patent. The “cleaning effect” occurs as a result of the centrifugal force pulling the collected oil out of the pores of the media radially outward of the filter element. By generating large enough centrifugal forces, one can theoretically extend filter life indefinitely. The present invention integrates a coalescing filter assembly with the rotating component of a bypass lube centrifuge, such that the blowby flow must pass through the spinning coalescing filter element prior to exhausting to the atmosphere or being fed back into the air intake system upstream of the air filter. The centrifugal force imparted to the oil collected within the coalescing filter element causes the separated oil to be rapidly expelled, as has been described in the '595 patent. The integration of the coalescing filter assembly with a centrifuge, according to the present invention, is seen as a novel and unobvious improvement to the current state of the art.